ios快速生成单例(宏)

 //.h
# define single_interface(className) + (className *)shared##className;
 
 //.m
 //  代表下一行也属于宏
 // ## 是分隔符
# define single_implementation(className) 
 static className *_instance; 
 
 + (className *)shared##className 
 { 
     if (_instance == nil) { 
         _instance = [[self alloc] init]; 
     } 
     return _instance; 
 } 
 
 + (id)allocWithZone:(NSZone *)zone 
 { 
     static dispatch_once_t onceToken; 
     dispatch_once(&onceToken, ^{    
     _instance = [super allocWithZone:zone]; 
     }); 
     return _instance; 
 }